1. Field of the Invention
The present invention relates to integrated circuit amplifiers, and more particularly, to such amplifiers having two signal inputs.
Reducing size while providing full performance capability is a major goal for a successful hearing aid. Achieving such a goal usually requires that the number of components in the hearing aid system be minimized. Of course, keeping the number of components in a hearing aid system small in number often conflicts with the desire to provide full capability.
Among the capabilities desired for hearing aids is the ability to use a "telephone coil", i.e. inductive sensors, therewith so that the hearing aid user can switch such a coil into the circuit to serve as the incoming information sensor for use with a telephone. On the other hand, such a coil is to be switched out of the circuit and a microphone switched into the circuit to sense the information in acoustic signals which are exchanged in ordinary face-to-face conversation. Another desirable capability is providing a tone control to enable adjusting the frequency content of that which is heard by the user of the hearing aid.
Such capabilities require provision of a coil to permit inductive sensing of telephone signals and a microphone to permit acoustic sensing. Tone control will require providing a potentiometer to permit varying circuit conditions to thereby control tone. In addition, coupling capacitors, to couple the coil and the microphone to the circuit, and a switch are needed. Further circuitry to accommodate these capabilities is often required.
These components are usually introduced into the hearing aid circuitry to interact therewith at the inputs to the amplification system. Thus, an amplification system formed in a monolithic integrated circuit having its initial portion containing as many of the circuit components as possible to accommodate such capabilities is needed to minimize the component count.